Olympic Viewing Guide: Phelps Retires on Action-Packed Day

With gold medal tennis and women's 100m, it's a busy day at London 2012

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Michael Phelps retires from swimming in the 4x100 individual medley relay in the afternoon; Usain Bolt, the world's fastest man, ran his first race of the Games and Serena Williams notched a gold in the women's championship tennis match.

Saturday is proving one of the busiest and most emotional days of the London 2012 Olympic Games.

Michael Phelps, the most decorated Olympian in history, competed in his final race during these Olympic games, earning a gold medal in the 4x100m medley relay. All eyes were on the Aquatics Centre pool, where Phelps swam the butterfly leg in the event, a fitting bookend to an Olympic career that began 12 years ago in Sydney, when he swam the 200m butterfly as a 15-year-old.

Possibly the greatest swimmer of all time, Phelps has achieved an unprecedented 22 Olympic medals, 18 golds, two silvers and two bronzes. He also holds two three-peat records following his showdown with rival teammate Ryan Lochte in the 200m individual medley on Thursday, and his win in the 100m butterfly final Friday.

Equally as riveting, Oscar Pistorius – the double-amputee South African "Blade Runner" – advanced to the 400m semifinals in his Olympic debut. Throw in the first major women's running race, and a gold for Serena Williams over Maria Sharapova, and it's fair to say London 2012 is in full swing.

Olympic Fever: Fans and Faces

Earlier in the morning, history was made at Olympic Stadium when 25-year-old Pistorius advanced to the 400m semifinals on his carbon-fiber prosthetic legs, as USA's gold medal favorite LaShawn Merritt re-injured himself in his heat, losing his shot at continuing.

Jamaican sprinter extraordinare Usain Bolt and his rival Yohan Blake also made their 2012 Olympic debut, both qualifying for the 100m semifinals alongside a trio of elite U.S. athletes—Justin Gatlin, Tyson Gay and Ryan Bailey.

On the grass of Wimbledon, Williams trounced Sharapova 6-0, 6-1 for the gold, notching her first Olympics singles title. Hours after winning her gold medal, Serena Williams returned to the court with sister Venus on Saturday afternoon to compete in the women's doubles semi-final round against Maria Kirilenko and Nadia Petrova of Russia. The United States' Bob and Mike Bryan also won gold in the men's doubles match over France's Michael Liodra and Jo-Wilfried Tsonga.

The defending Olympic champion U.S. men's volleyball team lost to Russia in a preliminary-round match on Saturday afternoon. The Americans came out strong on the court, winning the first two games 29-27, 25-19. But, the Russians fought back to take the next three games, 26-24, 25-16, and 15-8. The U.S. men will face Tunisia in their last preliminary-round match on August 6.